In prior spraying applications, when nozzles have been used without a check valve, the result at start up has been that fluid at first drips or dribbles from the nozzle, and then flows in an increasing stream which develops into a spray as full pump pressure develops. Only then is the desired spray pattern achieved. At shut down, a similar sequence occurs except in reverse order. Thus, when the pump stops the spray pattern deteriorates into a small fluid stream which issues from the nozzle for a short period and that stream reverts to a dribble as the pressure approaches zero. The presence of such streams or dribbles is, at the very least, wasteful of the fluid intended to be sprayed and often objectionable or even dangerous. For example, in the operation of oil burners, when fuel enters the combustion chamber in anything other than the desired spray pattern, the fuel is difficult to ignite, incomplete combustion occurs after ignition, undesired combustion products are produced and fuel is wasted. Another example is in agricultural spraying operations where insecticides, herbicides, disinfectants and other chemicals are used for a variety of purposes. Here, in addition to wastefulness, other potential problems include skin or foliage burns, eye irritation or injury and dangerous accumulation of chemicals on surfaces.
More recently, a variety of efforts have been made to improve these start up and shut down shortcomings by the use of check valves in combination with spray nozzles. In a typical prior check valve significant pressure drop occurs as a valve opens because applied pressure on the back of the movable valve member partially cancels supply pressure on the front. There are several potential problems caused by the pressure drop through such a typical check valve. It is necessary to increase system pressure to compensate for reduced flow due to the pressure required to hold the valve open. Failure of a typical check valve could result in a higher than specified pressure and flow condition. Removal of a check valve from a system without a system pressure adjustment could also result in a higher than specified pressure and flow condition. The operating or opening pressure of the typical check valve in a nozzle is usually significantly lower than the nozzle operating pressure in order to minimize pump capacity or pressure requirements. This lower check valve pressure reduces the effectiveness of the valve in controlling complete spray formation at start up and complete flow stoppage at shut down. Thus, for example, it may be necessary to supply 135 psi of pump pressure to hold such a valve open and to maintain 100 psi in the spray nozzle of a domestic oil burner. Thus the pressure drop is 35 psi. In this instance, since fluid flow through the nozzle does not begin until the pressure reaches 35 psi, the time interval to reach full pressure and full spray is somewhat reduced as compared to a nozzle without a valve and the time that the nozzle produces a drip or drizzle or small stream or undeveloped spray pattern is also reduced to a degree. While these check valve/nozzle combinations have achieved some reduction in the time interval required to reach full pressure and full spray, there is still an objectionable time lapse from the beginning of flow to the development of a full spray pattern. Conversely, when the pump is turned off, there is a reduced but still significant time before flow ceases entirely. Thus, unless a much larger pump is used, the same problems as recited above with simple nozzles still exist in valve/nozzle combinations except to a lesser degree.